I always find it weird that glass breaks at its resonant frequency and want to know why it causes it to break
In: Physics
Basically, as the material is vibrating it bends back and forth. When in resonance that back and forth movement is becoming slightly larger with every oscillation. Eventually that bending exceeds the strength of the material and it will crack or otherwise deform so that it is no longer in resonance.
Glass actually won’t do this, unless it is very well made. For something to shatter at a resonant frequency you need an essentially perfect crystalline structure. My understanding is that when the frequency of sound lines up with the length of those crystals it creates standing waves in the material, they get “pushed” farther and farther away from eachother eventually causing it to shatter
Objects will naturally vibrate at a certain frequency, which is dependent on the size , shape, and material that the object is made out of. If you’ve ever seen or used a tuning fork before, those are very precisely manufactured to have an exact frequency, and that frequency is what you hear when you hit them.
When you expose an object to a vibration, it will cause the object to vibrate, but the object will vibrate at its natural, or resonant frequency. If the vibration you expose it to is at a different frequency, then the 2 vibrations won’t line up closely, so the object will likely be fine. If you expose an object to a vibration that’s at it’s resonant frequency though, the vibrations will add up. It will start small, but every cycle of the vibration, the amplitude (how far the object moves) will keep growing. So it may start with a displacement of 0.0002 inches, and on the next cycle it will move 0.0005, then 0.001 inches, and just keep getting larger. The increasing displacement results in higher acceleration, and therefor higher forces on the object, until it just isn’t strong enough to keep itself together.
If you’re ever pushing someone on a swing, you know you want to give them a push when they’re on the way forward right? It’s a similar thing with materials and their resonant frequency.
When something is vibrating, the structure of what it’s made out of causes it to vibrate at a certain rate. Similarly, if you were singing a perfect note, that is causing the air to push in waves at a certain rate. If you line up those things perfectly, then each time the air pushes, it is in line with the object vibrating in one direction. This means the sound wave is helping to make the vibration bigger, just like pushing a swing at the right time. Eventually, if do this more and more, you can potentially ~~push them right over the top of the swingset~~ break the glass because it is vibrating itself apart.
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